1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer and the like, and more particularly, it relates to an image forming apparatus having an image bearing member capable of bearing an electrostatic latent image and toner image and a recording material bearing member capable of bearing a recording material.
2. Related Background Art
As an image forming apparatus having an image bearing member and a recording material bearing member, color image forming apparatuses are already known. Among such color image forming apparatuses, a color image forming apparatus wherein toner images having different colors formed on an image bearing member by charging, exposure and development are successively transferred onto a recording material in a superposed fashion to obtain a full-color image has been put to practical use.
An example of a four color image forming apparatus will be briefly explained with reference to FIG. 4.
This image forming apparatus includes a photosensitive drum 1 as an image bearing member that is rotatably supported. The photosensitive drum 1 is constituted by an aluminum cylinder having a diameter of about 40 mm, for example, and a photo-conductive layer made of organic photosensitive material (OPC) and coated on a peripheral surface of the cylinder. The photosensitive drum 1 is rotated by a drive means (not shown) in a direction shown by the arrow R1 at, for example, a peripheral speed of 100 mm/sec. Incidentally, the photo-conductive layer is not limited to OPC, but may be made of A--Si, CdS, Se or the like. Around the photosensitive drum 1, in order along a rotational direction thereof, there are arranged a charge device 2, an exposure device 3, a developing means 5, a transfer device 6, a cleaning device 7 and the like. The charge device 2 comprises a charge roller contacted with a surface of the photosensitive drum 1 and adapted to apply vias voltage, thereby uniformly charging the surface of the photosensitive drum 1. For example, the vias voltage obtained by overlapping DC voltage of -700 V to AC voltage having AC frequency of 1000 Hz and peak-to-peak voltage Vpp of 1500 V is applied to the charge roller, thereby uniformly charging the surface of the photosensitive drum 1 to -700 V.
The exposure device 3 is disposed above the photosensitive drum 1 and has an optical unit 10 including a laser diode for emitting laser light, a polygon mirror rotated by a high speed motor, a lens and the like, and a reflection mirror 11. For example, when a signal corresponding to image information for each color is inputted to the charge device, the charge device illuminates the laser light onto the surface of the photosensitive drum 1 through a light path E to change the voltage of the illuminated portion to about -100 V, thereby forming an electrostatic latent image
The developing means 5 serves to adhere toner to the electrostatic latent image and has a rotary member 13 rotatably supported on a shaft 12, and four developing devices (i.e. yellow developing device 15Y containing yellow toner, magenta developing device 15M containing magenta toner., cyan developing device 15C containing cyan toner and black developing device 15BK containing black toner) mounted on the rotary member 13. These four developing devices have substantially the same construction as each other. Explaining the yellow developing device 15Y shown in FIG. 5, the developing device 15Y comprises a coating roller 16, a developing roller 17 and a toner regulating member 19, so that, as the developing roller 17 is rotated, the toner is coated on the developing roller 17 by the coating roller 16, the thickness of a toner layer on the developing roller is regulated and the desired triboelectric brush is applied to the toner by the toner regulating member. The toner regulating member 19 may be made of material having charging polarity opposite to the charging polarity of toner (for example, nylon when the toner is to be negatively charged, or silicone rubber when the toner is to be positively charged).
Further, preferably, a peripheral speed of the developing roller 17 is selected to be greater than the peripheral speed of the photosensitive drum 1 by 1.0 to 2.0 times. Furthermore, as shown in FIG. 4, an opening 15a of each developing device is opposed to the surface of the photosensitive drum 1 when the desired developing device is brought to a developing position by rotating the rotary member 13. In the developing position, the toner from the developing device is transferred onto the electrostatic latent image on the photosensitive drum 1. Incidentally, a drive means for driving the rotary member 13 is fully described in the Japanese Patent Laid-open No. 50-93437, for example.
The transfer device 6 for transferring the toner image formed on the photosensitive drum 1 onto a recording material is disposed in a confronting relation to the developing means 5 with the interposition of the photosensitive drum 1. The transfer device 6 comprises a cylindrical transfer drum 20 rotatably supported for rotation in a direction shown by the arrow R2. The transfer drum 20 is constituted by a metallic cylinder 21 having a diameter of 156 mm, for example, an elastic layer 22 made of foam urethane and having a thickness of 2 mm and wound around the metallic cylinder, and a PVDF layer 23 having a thickness of 100 .mu.m and wound around the elastic layer. A recording material supplied from a sheet supply cassette 9 by a pick-up roller (not shown) is gripped by a gripper 25 and then is electrostatically wound around the surface of the transfer drum 20 by an absorb roller 26 to which voltage is applied. The toner image formed on the photosensitive drum 1 is transferred onto the recording material wound around the transfer drum by a transfer voltage from a power source (not shown).
The above-mentioned series of image forming processes from the charging step to the transferring step are repeated by four timers for different colors. That is to say, by such image forming processes, four toner images having different colors are transferred onto the recording material born by the transfer drum 20 in a superposed fashion. The recording material to which the toner images were transferred is separated from the transfer drum 20 by a separation pawl 30 and is sent to a fixing device 32 by a convey device 31. Then, the recording material is heated by the conventional fixing device 32 having a heat roller 32a and a pressure roller 32b, so that the toners are fused and mixed to form a permanent color image on the recording material. Then, the recording material is discharged onto a discharge tray (not shown) out of the apparatus.
On the other hand, the residual toner remaining on the photosensitive drum 1 is removed by the cleaning device 7 comprising a conventional fur brush, blade or the like. Further, the residual toner remaining on the transfer drum 20 is also removed by a transfer drum cleaning device 33 comprising a fur brush, web or the like, if necessary. Further, the transfer drum from which the residual toner was removed is initialized by removing the charge from the surface of the drum by means of an electricity removal roller 35.
Now, a fixing operation of the fixing device 32 will be fully explained. In a color image forming apparatus, not only natural color but also seven colors (such as red, green, blue and the like) cannot be obtained unless at least the yellow toner, magenta toner and cyan toner are fused and mixed. Thus, the toner must be of sharp melt type which can easily be mixed, and, in the fixing device 32, when the recording material is pinched between the upper fixing roller 32a and the lower pressure roller 32b, a fixing nip N2 for contacting the recording material with the rollers 32a, 32b must be widened sufficient to apply adequate heat amount to the toner. In addition, adequate pressure must be applied to easily mix the different color toners. More specifically, in a standard monochrome image forming apparatus, the pressure may be in the order of 5 to 10 kg; whereas, in the color image forming apparatus, the pressure is normally in the order of 30 to 50 kg.
However, in the above-mentioned image forming apparatus wherein the toner images are formed on the recording material wound around the recording material bearing member, the following problems arise:
First of all, as shown in FIG. 4, in an image forming apparatus wherein a distance between a transfer nip N1 and the fixing nip N2 becomes greater than a length of a recording material in a conveying direction so that the fixing operation is started after the transferring operation is completely finished, upon continuous print, if a next or second recording material and so on is miss-supplied or jammed due to the failure of the gripper, at this point, when the toner images are completely transferred to a first recording material and preferably is separated from the transfer drum 20 (that is, when a trailing end of the first recording material has passed through the transfer nip N1), a sheet supply portion, transfer drum 20 and the like are immediately stopped, and only the convey device 31 (between the transfer drum 20 and the fixing device 32) fixing device 32 and discharge device (not shown) are driven. The convey device, supply device, rollers and the like associated with the jam are immediately stopped in order to prevent the damage of the transfer drum 20 during the jam treatment and to improve the jam treating operability. In this way, if the second recording material is jammed, when the first recording material has passed through the transfer nip N1, the normal fixing operation can be effected regarding the first recording material, and the first recording material can be discharged as a correct copy sheet. That is to say, the number of recording materials which should be subjected to the jam treatment is minimized, thereby reducing the number of the waste recording materials and the jam treatment time.
Further, if the next or second recording material and so on is miss-supplied or jammed due to the failure of the gripper, when the transferring of the toner images regarding the first recording material is not completed, the sheet supply device, transfer drum 20, fixing device 32, discharge device and the like are immediately stopped. In such a case, since the first recording material is not yet sent to the fixing device 32, such recording material can easily be removed (jam treatment).
However, as shown in FIG. 6, when the apparatus is made compact by reducing the distance between the transfer nip N1 (referred to as "transfer point Q" hereinafter) and the fixing nip N2 (referred to as "fixing point S" hereinafter), upon the continuous print, if the next or second recording material and so on is miss-supplied or jammed due to the failure of the gripper, in almost cases, since the transferring of the toner images onto the first recording material is not completed and a tip end of the first recording material has already reached the fixing device 32, the first recording material not directly associating with the jam is in vain and this first recording material must also be removed during the jam treatment. That is to say, if the second recording material is jammed, the first recording material not directly associated with the jam must also be removed, and this removing operation is very dangerous-since the recording material should be removed from the fixing device 32 under high temperature and high pressure. Further, after the jam treatment, when the image formation is re-started, the image must be formed on a new recording material, thereby wasting the recording material and increasing the image forming time.